Rassputin said HB would bring about Ragnarok ,the apocalypse...but just because he didn't do it in the first film doesn't mean he still can't...perhaps his destiny as destroyer of the world was truly meant to be his fathering the engines of the destruction - his kids....

Liz will suffer more than anyone....this can go several ways - actual,physical suffering (indeed,i'd imagine her very life might be at risk,especially if they have big horns and stone hands in the womb) , or the suffering meaning knowing her kids are the most evil things in the history of our planet....

there could also be interesting subtext for this - HB and Liz at odds...one or both of your kids will destroy the Earth...how do you deal with that? if you see them turning evil,do you still love and protect them? do you kill them,knowing what will happen if you don't (including your own likely destruction)? i can see this being a MAJOR conflict between the 2,with one trying to protect them,no matter what they do...and one trying to neutralize the threat...imagine how HB would end up if he has to destroy one of his kids?